The objectives of this research are to provide a basic understanding of the synaptic mechanisms which control the development of the functional circuitry of the cerebral cortex. Particularly, the neural mechanisms that underlie amblyopia or the group of visual disturbances that belie any organic explanation at the level of the eye. For the first time, a combination of technology can allow a direct analysis of these changes at the level of the geniculocortical synapse (site of innervation of visual cortical neurons by axons from the dorsal lateral geniculate nucleus (LGN/d)). During the last project period, individual physiologically classified geniculocortical axons from kittens and adult cats reared with monocular patterned-vision deprivation (MD) were successfully filled (intra-axonally) with the marker enzymes horseradish peroxidase (HRP) and the distribution of their axonal arborizations and synaptic boutons were quantitatively analyzed in 3-dimensional space at the light microscope level. All of this material was analyzed and prepared in such a manner to now be available for electron microscopic study of the synaptic architecture of these identified axons. By studying the nature and degree of synaptic divergence of individual boutons on single axonal arborizations, the effect of development and unequal binocular inputs on the elaboration of cortical microcircuitry will be tested. The functional extent of spatial overlap of single right and left eye geniculocortical afferents and their activity distribution profile in visual cortex will be evaluated by recording the average electrophysiological activity from a fixed array of cortical electrodes in response to activation of individual neighboring right and left eye activated LGNd neurons in kittens, MD and normal adult cats. This will allow a functional evaluation of the spatial interactions and efficacy of populations of neighboring right and left eye afferents in individual animals in order to investigate the basis of functional binocular interactions, in vivo. Moreover, we have developed an in vitro system to evaluate the capacity for expression of axonal arborizations of LGNd neurons from postnatal animals that have developed through the critical period with MD. These experiments will test for the mechanisms of unequal binocular activity effects on re-expression of innervation patterns in a strictly controlled environment.